Microwave magneto-acoustic delay line



Jan. 30, 1968 w, J D R JR" ET AL 3,366,896

MICROWAVE MAGNETO-ACOUSTIC DELAYLINE Filed April 8, 1966 FIG. INPUT ou rPu-T INVENTORS,

. BY ROBERT H. PROAI M 7%. W 1. W M

g ATTORNEYS WILLIAM J. SKUDERA,JR,-, I

3,366,896 MICROWAVE MAGNETOACOUSTIC DELAY LINE William J. Skudera, in, Eatontown, and Robert H. Sproat,

fiakhurst, Nl, assignors to the United States of America as represented by the Secretary of the Army Filed Apr. 8, 1966, Ser. No. 541,403 8 Claims. (Cl. 333-30) ABSTRACT OF THE DISCLQSURE This delay line comprises an elongated composite rod comprising a non-magnetic central portion of good acoustic transmission properties with ferrimagnetic crystals bonded to either end thereof. Input and output coupling wires are located near or at the bonded interfaces. The ferrimagnetic crystals act as transducers for converting the input microwave energy to acoustic energy at the input end of the device and vice versa at the output end. The time delay can be varied by varying the magnitude of the axial magnetic field.

The present invention relates to magneto-acoustic delay lines and more particularly to a novel delay line of this type which combines the advantageous features of long and variable time delay, wide band operation, low distortion and low insertion loss. In microwave delay lines, in general, a microwave applied signal is converted to an acoustic or elastic wave of corresponding frequency and propagates down a delay medium which has good acoustic transmission properties. At the output end the delay acoustic Wave is converted back to electromagnetic form. These conversions may be accomplished by means of piezoelectric transducers mounted at either end of the delay medium, or in the case of a magneto-acoustic delay line, the interaction of the applied microwave signal at the input end of a monocrystalline ferrirnagnetic rod with a generally axial direct magnetic field generates spin waves which, after traveling for a short distance within the crystal, are converted to acoustic waves or phonons. Upon reaching the output end of the crystal the reverse process takes place and the phonons are reconverted to magnetic spin waves and then coupled out as delayed electromagnetic energy. Prior art delay lines of the magneto-acoustic type have comprised an elongated rod of monocrystalline ferrimagnetic material, for example, yttrium-iron-garnet, with a generally axial magnetic biasing field and a pair of coupling Wires adjacent to or abutting the opposite ends of the rod to form input and output connections. The ends of the rod must be polished optically flat and parallel because the phonons, generated within the crystal near the input end thereof, first travel toward the input end face and are reflected therefrom to the opposite or output end face where a second reflection takes place. In traveling back toward the input end face, the phonons are reconverted to spin waves and coupled out of the crystal as delayed electromagnetic energy. The propagational velocity of the spin waves in such a delay line is a function of the applied magnetic field and therefore the overall delay can be changed by varying the field intensity. The overall delay however is limited by the fact that monocrystalline ferrimagnetic materials cannot be produced economically in lengths much over half an inch. In accordance with the present invention, a magnetoacoustic delay line with longer time delays than heretofore possible is provided by bonding a pair of ferrimagnetic crystals to the ends of a monocrystalline delay medium which has good acoustic transmission properties. By applying the input and output coupling wires at or near the bonded interfaces near each end of the delay medium, low insertion loss has been achieved. The con- 3,3h6,8% Patented Jan. 30, 1968 nection of the coupling wires to the device is critical and several different ways of accomplishing this connection are disclosed herein. The two ferrimagnetic crystals function as transducers for converting the input microwave energy to acoustic or phonon energy at the input and vice-versa at the output. The use of the magneticacoustic ferrimagnetic crystals as transducers has the advantage of variable time delay operation mentioned above and broadband operation. Conventional transducers such as quartz piezoelectric crystals do not have these advantages.

It is therefore an object of this invention to provide an improved microwave delay line of the magneto-acoustic type.

A further object of the invention is to provide a microwave delay line with long but variable delay, low insertion loss, broadband operation and low distortion.

These and other advantages and objects of the invention will become apparent from the following detailed description and drawings, in which:

FIG. 1 is a cross-sectional view of an illustrative delay line embodying the present invention.

FIG. 2 is a diagram illustrating the principle of operation of the delay line of FIG. 1.

FIGS. 3, 4A-4E and 5 show several ways in which the coupling wires may be connected to the crystal interfaces.

Referring first to FIG. 1, the delay line shown therein comprises an elongated composite structure 13 which may conveniently be cylindrical in shape composed of a monocrystalline acoustic delay medium 17 with monocrystalline magnetic-acoustic transducers 15 and 19 bonded to each end thereof. The delay line 13 is supported in a holder 6, which is made of a non-magnetic metal such as brass. A generally axial direct magnetic field H is applied from an external source. The microwave energy to be delayed is applied at input coaxial connector 5. The input coupling wire 7 is an extension of the inner conductor of the coaxial connector and extends downward and through the delay line at or in the vicinity of the interface 23 between the delay medium 17 and the input magneto-acoustic transducer 19. The coupling wire is grounded at its lower end to the holder 6. The arrangement of the output coupling wire 11 relative to the interface 21 between the delay medium 17 and the output magneto-acoustic transducer 15 is the same as that of the input coupling wire. By placing the coupling wires at the interfaces as shown rather than at the extreme ends 14 and 18 of the composite delay line, lower insertion loss is attained because no reflection of the waves from these extreme ends of the device is necessary. FIG. 2 illustrates the path of the magnetic spin waves and acoustic waves or phonons within the delay line of FIG. 1 the same reference numbers here designating corresponding parts. Electromagnetic energy from the input coupling wire 7 is converted to magnetic spin waves at the turning point, TF1 of the input transducer. Since the transducer 19 is ferrimagnetic and the delay medium 17 is non-magnetic the magnetic field drops off in intensity from the point TPl to the interface 23. With such a field gradient the magnetic spin waves travel from TPl toward the interface 23, and in so doing pass the crossover region CR1, where they are converted to acoustic waves or phonons of the same frequency. These phonons then continue toward the right across both interfaces in the direction of the arrow, Within the output transducer 15 a similar drop off of magnetic field toward the interface 21 occurs with the result that the crossover region CR2 and the turning point TPZ are positioned as shown. The phonons are converted back to magnetic spin waves at CR2 and proceed to TF2 where they are coupled out via output coupling wire 11 as delayed electromagnetic energy. While in the spin wave state the propagational velocity of the Wave depends on the magnetic field strength and therefore the overall delay can be controlled by varying H. The propagational velocity within the delay medium 17 is independent of magnetic field and depends on its physical properties and is generally less than centimeters/second. The delay medium should be chosen from the crystalline materials with a low temperature cocfficient of velocity so that the delay will not be temperature sensitive and for low phonon attenuation or low insertion loss. Monocrystalline Z cut quartz, ruby or sapphire are examples of good delay media.

In order for the phonons to pass across the interfaces without distortion or dispersion, the interfaces must be made extremely thin and the abutting ends of the transducers and the delay medium must be polished optically flat and parallel. The coupling wires in the interface region may take the form of conductive materials deposited by evaporation, sputtering, or printed circuit techniques over part or all of the ends of the delay medium or the abutting end of either transducer. Several configurations of these deposited coupling wires are shown in FIGS. 4A-4E, the conductive coupling wire being indicated by the regions 25, 27, 29, 31 and 33 respectively. The conductors extend around to the curved surface of the delay medium to form a terminal for the connection of the remainder of the coupling wires 7 or 11 thereto, as indicated at 28 in FIG. 4A. In the embodiments of FIGS. 4A, 43, 4D and 4E the portion 26 of the end of the delay medium 17 not covered by the coupling wire may be coated with bonding material of a thickness the same as the deposited coupling wire, thus minimizing the thickness of the interface. In the embodiment of FIG. 4C the conductor 29 covers the entire end of delay medium 17. In this case the coupling wire 29 may itself have both conductive and bonding properties resulting in a maximum area for the coupling Wire, and also minimizing interface thickness. Optical bonds, in which the highly polished end faces of the two crystals adhere to each other without any bonding material, may also be used. Different ones of the shapes of the coupling wires in FIGS. 4A4E will give better performance in different frequency ranges and will provide more or less coupling between the input and output circuits and the magnetoacoustic crystal, and also provide different impedance levels.

FIG. 3 shows another embodiment in which the coupling wires 7 and 11 are passed through diametral holes 35 and 37 near the ends of the delay medium 17. In the embodiment of FIG. 5, the coupling wires are placed in a diametral channel 39 cut in the end face of the delay medium 17. A conductive filler material may be used to fill up the groove or channel 39 after the coupling wire is placed therein. This in effect increases the surface area of the coupling wire and improves its coupling to the adjacent transducer.

While the invention has been described in connection with illustrative embodiments, the inventive concepts disclosed herein are of general application and hence the invention should be limited only by the scope of the appended claims.

What is claimed is:

1. A microwave delay line comprising a composite elongated rod comprising a central crystalline portion which has good microwave acoustic transmission properties and which is non-magnetic, and input and output magneto-acoustic transducers bonded to each end of said central portion, input and output coupling wires passing through said delay line in the vicinity of the interfaces between said central portion and said transducers, and means to produce a direct magnetic field generally axially along said rod.

2. The delay line of claim 1 wherein said coupling wires in passing through said delay line comprise a thin film of conductive material deposited on one of the abutting ends of said central portion of said transducer.

3. The delay line of claim 1 wherein said coupling wires pass through diametral holes near opposite ends of said central portion.

4. The delay line of claim 1 wherein said coupling wires pass through diametral channels in the ends of said central portion.

5. A microwave delay line comprising a composite elongated rod comprising a central acoustic delay medi um, which is non-magnetic, with ferrimagnetic magnetoacoustic transducers bonded to opposite ends thereof, a magnetic field passing generally axially through said rod, and means to apply input microwave energy to one of the interfaces of one of said transducers and one end of said delay medium and to withdraw delayed microwave energy from the interface of the other of said transducers and the opposite end of said delay medium.

6. The delay line of claim 5 wherein said last-named means comprises a coupling wire passing through each of said interfaces.

7. The delay line of claim 5 wherein said last-named means comprises a coupling wire passing through said delay line in the vicinity of each of said interfaces.

8. The delay line of claim 5 wherein the interfaces between said delay medium and said transducers are made extremely thin and the abutting ends of said delay medium and said transducers are optically fiat and parallel.

No references cited.

ROY LAKE, Primary Examiner.

DARWIN R. HOSTETTER, Examiner. 

